FROM THE SPRING TO THE BASIN: THE INFLUENCE OF RECHARGE PATTERNS ON AQUIFER REDOX STATE AND CONTAMINANT TRANSPORT

Water quality has long been known to degrade when contaminated surface water intrudes karst aquifers; however, water quality may also degrade when surface water intrusion alters aquifer redox state. Aquifer redox state controls the fate and mobility of many common karst contaminants (trace metals, nitrate, and organic compounds); therefore managing water quality requires understanding both surface fluxes of contaminants and changes to contaminant mobility in the aquifer due to changes in redox state. Surface water intrusion is common in the Suwannee River basin, where the eogenetic Upper Floridan aquifer transitions from confined to unconfined, and numerous springs discharge to the river. Intrusions are regulated by subtle shifts in the timing, magnitude, and location of rainfall, which alters the source of aquifer contamination and changes the aquifer redox state. We developed a basin-wide conceptual model for how shifting rainfall patterns affect intrusion and contaminant mobilization by integrating rainfall patterns, hydraulic head response, and seasonal changes in surface water chemistry with high resolution measurements of water composition through river intrusion events. At one extreme, rain on the confined portion of the aquifer increases river stage without increasing the hydraulic head in the unconfined portion of the aquifer, and river water penetrates deeply into the aquifer carrying both organic carbon and trace metals. Oxidation of intruded organic carbon produces extended reducing conditions in the aquifer. The magnitude of the reducing conditions in the aquifer is controlled by seasonal differences in river water chemistry. At the other extreme, rain over the unconfined portion of the river basin increases aquifer hydraulic head, which remains above river stage, preventing river intrusion. Contamination comes from diffuse recharge through the land surface, and produces oxidizing conditions in the aquifer that may limit denitrification. Most storms fall between these two extremes, with impacts to the aquifer from both diffuse recharge and focused intrusion of river water. Predicting and mitigating contamination in the aquifer requires an understanding of how shifts in the location, intensity, and frequency of precipitation alter contaminant transport and aquifer redox state.